Radiotherapy involves the production of a beam of ionising radiation, usually x-rays or a beam of electrons or other sub-atomic particles. This is directed towards a target region of the patient, and adversely affects the target cells (typically tumour cells) causing an alleviation of the patient's symptoms. Generally, it is preferred to delimit the radiation beam so that the dose is maximised in the target cells and minimised in the healthy cells of the patient, as this improves the efficiency of treatment and reduces the side effects suffered by the patient. For example, the radiation beam may be shaped to conform to the cross-section of the target region.
One principal component in delimiting the radiation dose is the so-called “multi-leaf collimator” (MLC). This is a collimator which sits inside the radiation head of the therapeutic system, and consists of a large number of elongate thin leaves arranged side by side laterally in an array. Each leaf is moveable longitudinally so that its tip can be extended into or withdrawn from a radiation field. The leaves can thus be positioned so as to define a variable edge to the beam of radiation, and this is used to impart a variable edge to the radiation beam passing through the radiation field. All the leaves can be withdrawn entirely to open the radiation field (even if in practice this should never occur during operation), or all the leaves can be extended to their fullest extent so as to close it down. Alternatively, some leaves can be withdrawn and some extended so as to define any desired shape, within operational limits. A multi-leaf collimator usually consists of two banks of such arrays, each bank projecting into the radiation field from opposite sides of the collimator.
The depth of each leaf is one of the parameters which defines the leaf's ability to mitigate (i.e. block) the radiation beam passing through the window. The material of manufacture also plays a part, and for this reason each leaf is typically manufactured from an element with high atomic number, such as tungsten. However, even using such materials, each leaf must have a significant depth in the direction of the beam in order to adequately block the high-energy radiation used in radiotherapy (where photons usually have energies in the megavolt range). Most leaves have a depth of between 60 and 120 mm, but in practice the deeper a leaf is, the more effective it will be in blocking and shaping the radiation.
In order to achieve a high resolution when collimating the radiation beam, each leaf should also be relatively thin in the lateral direction. That is, the tips of the leaves in the array collectively define an edge of the radiation beam. If each leaf is made as thin as possible, a greater number of leaves are used to define the edge and thus the shape of the radiation beam can be defined at a higher resolution.
Of course, the leaves on the MLC leaf bank need to be driven in some way. Given the design parameters set out above (i.e. narrow leaves arranged closely together, heavy materials, significant depth etc) this is no trivial task. Typically, this is by a series of lead screws connected to geared electric motors. The leaves are fitted with a small captive nut in which the lead screws fit, and the electric motors are fixed on a mounting plate directly behind the leaves. Rotation of the leadscrew by the motor therefore creates a linear movement of the leaf.
Our earlier application, WO 2009/129817, describes an improvement to this design in which each leaf has a lug which extends above or below the leaf, i.e. transverse to the lateral and longitudinal directions. The lug engages with a leadscrew which is itself driven by a motor. The set of motors for each leaf bank can thus sit above or below the banks of leaves rather than behind or to the side of the leaves.
However, in both prior designs the motors are arranged to the side of the leaf array. Thus a large amount of space in the radiation head is taken up by the motors rather than the leaves. If the motors could be made more compact, the depth of the leaves could be increased to take up the available space in the radiation head, in turn leading to an increase in the radiation-blocking effect of the collimator.